Analysis of Spaceflight-Associated Biometric and Refractive Changes in Astronauts.

Abstract

Purpose: To study changes in ocular biometry and refraction in the largest cohort to date of astronauts who have experienced long-duration spaceflight on the International Space Station (ISS).

Design: A prospective cohort study.

Participants: 29 astronauts METHODS: Preflight and postflight cycloplegic refraction and ocular biometry measurements were obtained from 56 eyes among 29 subjects. For each eye, the preflight-to-postflight changes in spherical equivalent (SE), axial length (AL), average corneal curvature (K), and anterior chamber depth (ACD) were calculated. The Fyodorov and Olsen-C formulas were used to estimate the relative contribution of each biometric parameter individually to the total change in SE. A linear mixed-model approach was used to assess the relationships between refraction measurements, biometric parameters, optic disc edema, and duration on the ISS.

Main outcome measures: Preflight-to-postflight changes in spherical equivalent, axial length, average corneal curvature, and anterior chamber depth.

Results: 27/56 (48.2%) eyes underwent a hyperopic shift, 8/56 (14.3%) underwent a myopic shift, and 21/56 (37.5%) eyes had no measurable change in SE. On average, this equated to a mild hyperopic shift of +0.12 D (95% CI, +0.02 to +0.22 D) that arose from a decrease in AL of -0.09 mm (95% CI, -0.14 to -0.04 mm), mitigated by a shortening in ACD of -0.09 mm (95% CI, -0.12 to -0.06 mm). Changes in K were variable and had little contribution to SE changes at the group level but often showed substantial change at the individual level. Statistical modeling revealed the greatest predictor for refractive change was baseline preflight refraction (P = 0.034), with myopic individuals experiencing the largest hyperopic shifts (and never a myopic shift) and baseline hyperopic individuals experiencing variable myopic to mildly hyperopic shifts.

Conclusions: Spaceflight is associated with decreases in AL and ACD and variable changes in K. On average, these changes result in a mild hyperopic shift in SE, although myopic shifts can be observed at the individual level. Prior reports of greater hyperopic shift may be a result of subjects being more myopic at baseline.